`On Mon, Dec 3, 2018 at 10:15 AM Hans Verkuil <hverkuil@xs4all.nl> wrote:>> On 12/02/2018 09:43 PM, Arnd Bergmann wrote:> > On Sun, Dec 2, 2018 at 2:47 PM Gabriel Francisco Mandaji> > <gfmandaji@gmail.com> wrote:> >> >> @@ -667,10 +653,28 @@ static void vivid_overlay(struct vivid_dev *dev, struct vivid_buffer *buf)> >>         }> >>  }> >>> >> +static void vivid_cap_update_frame_period(struct vivid_dev *dev)> >> +{> >> +       u64 f_period;> >> +> >> +       f_period = (u64)dev->timeperframe_vid_cap.numerator * 1000000000;> >> +       do_div(f_period, dev->timeperframe_vid_cap.denominator);> >> +       if (dev->field_cap == V4L2_FIELD_ALTERNATE)> >> +               do_div(f_period, 2);> >> +       /*> >> +        * If "End of Frame", then offset the exposure time by 0.9> >> +        * of the frame period.> >> +        */> >> +       dev->cap_frame_eof_offset = f_period * 9;> >> +       do_div(dev->cap_frame_eof_offset, 10);> >> +       dev->cap_frame_period = f_period;> >> +}> >> > Doing two or three do_div() operations is going to make this rather> > expensive on 32-bit architectures, and it looks like this happens for> > each frame?> >> > Since each one is a multiplication followed by a division, could this> > be changed to using a different factor followed by a bit shift?>> The division by 2 can obviously be replaced by a shift, and the> 'End of Frame' calculation can be simplified as well by multiplying by> 7 and dividing by 8 (again a simple shift): this equals 0.875 which is> close enough to 0.9 (so update the comment as well).The first division      f_period = (u64)dev->timeperframe_vid_cap.numerator * 1000000000;      do_div(f_period, dev->timeperframe_vid_cap.denominator);looks like it could be replaced with a fixed multiplication with aprecomputed 'u64 factor = div_u64(numerator * 100000000, denominator)> It's all a bit overkill since this function isn't called very often,> but these are easy changes to make.Ah, I assumed it was called once per frame or more. If this is only doneduring initalization, it doesn't matter.        Arnd`